I do the same thing. The best you can do is to keep the food dry, and
to sweep up crumbs every day.

I hear ya, but the flies are coming from somewhere, the cat food is not
the source...You could also elevate the food onto say a porch table and
put a box fan blowing across the food...for a quick fix.

but the cat food is not the source, not that many flies, flies are
nasty they blo anything.

Look around your property, like neighbors dogs...a group of trash
cans...there's maggots somewhere

Bleach is very effective for localized sterilisation. porch, stairs,
patio...watch out for bleach on wood, I almost broke my neck, bleach on
wood is veeery slippery.

There's yard foggers, perimeter sprays (lowes sells a gallon with a
retractable nozzle)...never used it, but it claims to eliminate flying
insects wherever you spray...probably a good 2-4 day effect. Course
check label for pet warnings.

The cats that you are feeding, are they breeding more stray cats
which are often times destined for a very ugly life and death?

If you cannot have them neutered and spade, maybe best is to avoid
breeding them. Maybe providing fresh/clean water is OK, at least
it does not attract flies.

Good luck.

Keep the food inside the house.